1. Field of the Invention
The present invention concerns an analog-to-digital and digital-to-analog converter or codec (encoder/decoder), in particular a converter made in the form of a discrete component. The present invention concerns notably a codec designed for the acquisition and restitution of voice signals in an electrically autonomous device.
2. Background of the Related Art
Many known apparatus require a voice signal acquisition and restitution system. By way of non-limiting examples, one can mention portable or cordless telephones, hearing aids, digital recording apparatus such as Dictaphones, voice-controlled apparatus, headphones, etc. These apparatus have in common an analog-to-digital conversion circuit capable of converting the analog signals supplied by a transducer, for example a microphone, into digital signals processed by a microprocessor or a signal processor (DSP). If the apparatus is designed to restitute sounds generated or processed by the processor, it also comprises a digital-to-analog conversion circuit for converting the microprocessor""s digital signals into analog signals capable of driving a restitution element, for example a loudspeaker. It is known to integrate in a same discrete component the analog-to-digital conversion circuit and the digital-to-analog conversion circuit of an apparatus: this is called a codec (encoder/decoder).
In order to reduce the rate of the digital data at the output of the signal processor and thus to economize the pass-band of a transmission channel or the storage space of a data carrier, it is already known to make or program the microprocessor so that it interrupts the signal processing when no voice signal is picked up by the microphone. It has in fact been observed in a typical telephony application that the speaking time of each interlocutor is less than 50% of the total communication time. This characteristic of speech signals has been used for compressing the transmitted digital data.
For this purpose, signal processors are known that comprise a circuit or computer module allowing the presence or absence of voice to be detected and the transmission or storing of the data to be interrupted in the absence of voice.
The signal processor must however remain permanently powered in order to effect this detection, which proves particularly disadvantageous in a device having a low electrical autonomy. Furthermore, most of the processors commercially available on the market are not provided with a voice detection circuit, which considerably restricts the choice of available processors that can be used for a given application or requires this function to be realized by programming it on an existing microprocessor. A voice detection module realized by a software must however process the data from the codec in real time and therefore strongly burdens the microprocessor""s memory and its computational capacity.
Circuits are also known that are made in the form of discrete components that supply a signal indicating the absence or presence of voice in a flux of analog or digital data. The algorithms and the circuits used in this type of specialized circuits are however complex and allow notably to distinguish between a silence, a voice or a noise, for example by measuring the energy or power levels in one or several frequency bands. Such circuits are thus relatively expensive, complex to use and have a high electricity consumption. Furthermore, the presence of an additional discrete electronic circuit between the codec and the microprocessor is often undesirable in miniaturized portable devices. Finally, these circuits are not adapted for processing digital signals transmitted serially between the codec and the digital processor.
It is therefore an aim of the present invention to avoid the aforementioned disadvantages.
According to the invention, these aims are achieved by means of an analog-to-digital and/or digital-to-analog converter having the characteristics of the independent claims, preferred embodiments being furthermore mentioned in the dependent claims.
In particular, these aims are achieved by means of an analog-to-digital converter made in the form of a discrete electronic component, comprising notably a signal detection circuit for detecting and indicating outside the component the presence or absence of significant data in the sequence of digital samples generated by the conversion circuit.
By integrating the signal detection circuit into the analog-to-digital converter rather than into the signal processor, it is thus possible to generate a standby-mode signal that can be used for selectively deactivating the components downstream of the converter, notably the processor itself. This solution furthermore allows the signal detection circuit to process the converted data digitally and in parallel; it is thus possible to make a very simple circuit, requiring only a small silicon surface and having a reduced electrical consumption.
The signal detection circuit can for example comprise a digital comparator for comparing each sample of the digital sequence with a threshold value. Such a comparator can be made with very reduced surface requirements. In order to avoid overly quick variations of the standby-mode signal, an accumulator is preferably provided for integrating the signal at the output of the digital comparator. Such an accumulator can be made economically by means of a simple counter.
The aims of the invention are also achieved by means of an analog-to-digital converter made in the form of a discrete electronic component, comprising notably a signal detection circuit for detecting and indicating outside the component the presence or absence of significant data in the sequence of digital samples received.